(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

264                                ALASKAN GLACIER STUDIES
170 feet, trees being overturned and rocks overridden by the spreading ice. There was more wood than dirt and stone in the marginal debris heaped up by the spreading ice as it advanced into the forest. From Gilbert's photographic site on the rocky point to the south (G. Map 6) the evidence of the spreading was clearly seen and it was also plain that thickening had continued from 1909 to 1910, the amount being estimated as at least 60 to 75 feet.
Besides this spreading there was forward movement at the western margin from 1899 to 1910, for Gilbert says that in 1899 the belt of bare rock between the western margin of the glacier and the forest "was strewn with fragments not only of rock but also of wood, and trees were freshly overthrown at the margin of the forest. At the time of its attack on the forest the ice must have been 100 feet deeper than in the summer of 1899, and it also extended farther southward, as shown by a push moraine of rock at the water margin, 800 feet from the ice front. A second push moraine, less massive than the first, lay within it, being 160 feet from it at the water margin and elsewhere nearer to it than to the ice." He shows later, by the evidence of vegetation within this push-moraine along another part of the glacier, that this earlier advance had not taken place more recently than 1892 and perhaps in that year.
In June, 1909, Grant and Higgins found that the western margin had spread laterally far enough to encroach upon the forest and had advanced southward a distance estimated as about 500 feet beyond the position in 1899.1 This advance had taken place mainly since Grant's earlier visit in July, 1908.
In August, 1909, we found tha't the western margin of the glacier had not only moved forward the eight hundred feet from the 1899 terminus to the outer push moraine, but that the front had also moved forward approximately four hundred feet more. The advance had without doubt been even more than the twelve hundred feet from the 1899 to the 1909 terminus, for the glacier surely continued to retreat between 1899 and the period between 1905 and 1908, when the present advance began, judging by its behavior on Heather Island during this period. We cannot determine how much this recession amounted to, but whatever it was must be added to the 1200 feet of net advance between 1899 and 1909. If Grant's estimate of the position in June, 1909, is correct, the advance between June and August, 1909, was about 700 feet.
In overriding the barren area strewn with rock and wood on the western margin, the two push moraines, mentioned by Gilbert, had been destroyed but a new push moraine had been formed in 1909, rising ten or fifteen feet above a beach where there was no such feature ten years before. It was made up of beach rubble and the ice was still in active contact with it on August 28, although the moraine was not visibly shoved while we were upon it. Above it rose the debris-stained, crevasse-riven ice cliff, from whose slope large blocks of ice had recently fallen upon the surface of the push moraine. The surface of this moraine was five or six feet wide and nearly flat-topped. It had a steep outer slope upon which several young spruces were still growing, having been tilted from a vertical position to an angle of forty-five degrees (PI. XCVII, B), and in some cases even to a horizontal position, by the glacier pushing up the ground in which they grew.
The push moraine disappeared a few hundred feet back from the beach where the glacier was actively advancing into the forest along the western margin. Here we found a
i Grant, TL S. and Higgins, D. P., Glaciers of the Northern Part of Prince William Sound, Bull. Amer. Geog. Soc., Vol. XT.TT, 1910. p. 729. ,